In general, the surface of a solid base material such as a metal or polymer has a specific surface energy. The specific surface energy appears as a contact angle between a liquid and a solid when the liquid contacts the solid. Here, the liquid generally refers to water or oil, etc., and water shall be representatively mentioned as the liquid hereinafter. If the contact angle is smaller than 90° a spherical water drop loses its form on the surface of the solid to wet the surface of the solid, exhibiting wettability. If, however, the contact angle is larger than 90° the spherical water drop maintains its form on the surface of the solid to easily flow by an external force rather than wet the surface of the solid, exhibiting non-wettability. For example, if a water drop falls on a lotus leaf, it does not wet the lotus leaf but flows on the surface of the lotus leaf. This phenomenon indicates the non-wettability.
The value of the specific contact angle on the surface of the solid base material may change if the surface is processed to have fine protrusions and depressions (unevenness). Namely, a hydrophilic surface having a contact angle of smaller than 90 can have greater wettability through surface processing, and a hydrophobic surface having a contact angle of larger than 90 can have greater non-wettability through surface processing. The hydrophobic surface of the solid base material may be applicable in various manners. If the hydrophobic surface is applied to a piping structure, flow of liquid in a pipe can be facilitated to increase the quantity of flow and flow velocity. Thus, the application of the hydrophobic surface to a water service pipe or a boiler pipe can considerably reduce accumulation of foreign substances compared with the related art. In addition, if a polymer material is used for the hydrophobic surface, the inner surface of the pipe can be prevented from being corroded, and accordingly, water pollution can be reduced.
However, a technique for changing the contact angle on the surface of the solid for an arbitrary purpose is known, to date, as a MEMS (Micro Electro Mechanical Systems) process to which a semiconductor fabrication technique is applied, whereby the surface of the solid is formed to have fine protrusions and depressions of micro- or nano-scale. The MEMS process is an up-to-date technique employing the semi-conductor technique by mechanical engineering, but the semiconductor process incurs much costs. Namely, in order to form the protrusions and depressions of a nano-scale on the surface of the solid by using the MEMS process, operations such as oxidizing the metal surface, applying certain temperatures and certain voltages, and oxidizing and etching in a special solution are performed. The MEMS process cannot be performed in a general working environment but should be performed in a specially fabricated clean room, and machines required for the operation are high-priced equipment. In addition, the MEMS process is disadvantageous in that it cannot process a large surface at one time.
Thus, because the existing technique for forming the hydrophobic surface has a very complicated process, is not suitable for mass-production, and incurs high fabrication costs, its application is not easy.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.